The invention relates generally to an electrophysiological ("EP") apparatus for providing energy to biological tissue, and more particularly, to a catheter having a distal end region constructed for distributing axial forces applied to the catheter, in the distal direction, over an increased surface area relative to the surface area of the distal tip of the catheter.
The heart beat in a healthy human is controlled by the sinoatrial node ("S-A node") located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node ("A-V node") which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as "cardiac arrhythmia."
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation ("AF"), a procedure published by Cox et al. and known as the "Maze procedure" involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system.
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In the bipolar method, the flux traveling between the two electrodes of the catheter enters the tissue to cause ablation.
During ablation, the electrodes are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48.degree. Centigrade. Tissue heated to a temperature above 48.degree. C. becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37.degree. C., fairly uniformly to an ablation temperature above 48.degree. C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100.degree. C.
A basic configuration of an ablation catheter for applying RF energy includes a distal tip which is fitted with an electrode device. The electrode device is the source of an electrical signal that causes heating of the contacting and neighboring tissue. In the unipolar method, the electrode device may include a single electrode used for emitting RF energy. This single electrode acts as one electrical pole. The other electrical pole is formed by the backplate in contact with a patient's external body part. A RF source is applied to the electrode. The RF source is typically in the 500 kHz region and produces a sinusoidal voltage. When this is delivered between the distal tip of a standard electrode catheter and a backplate, it produces a localized RF heating effect and produces a well defined, deep acute lesion slightly larger than the tip electrode.
In other techniques, used in the treatment of atrial fibrillation, a plurality of spaced apart electrodes are located at the distal end of the catheter. RF energy is applied by the electrodes to the heart tissue to produce a long lesion. In an attempt to ensure intimate contact between the electrode and the target tissue the distal end of the catheter may have a preformed shape complementary or almost complementary to the expected heart tissue shape. To maintain such a shape a preformed stylet may be placed within the catheter. Typically the stylet is formed of a relatively large diameter wire having sufficient strength so that when the catheter is placed in the targeted anatomical location the catheter adequately resists unwanted distortion. The large diameter stylet also gives the catheter sufficient rigidity so that it can be satisfactorily manipulated by pushing, torquing etc. in order to improve electrode contact as needed. However, this increased rigidity may permit the application of higher axial forces to the tissue by the distal tip of the catheter. If the distal tip were in end-on contact with the heart tissue, axial forces applied to the proximal end of the catheter external to the patient may be transferred to the heart tissue by the more rigid catheter shaft through the end-on contact. Should an end-on contact of the distal tip to heart tissue occur while at the same time significant axial force is applied, heart tissue may be adversely affected. Thus, it is desirable to limit the amount of axial force that can be applied to heart tissue by the catheter in an end-on mode.
Hence, those skilled in the art have recognized a need for a catheter having a distal end region with sufficient rigidity such that the distal end region may be introduced into a biological cavity and intimate contact maintained between the electrodes of the distal end region and the tissue while at the same time having a structure such that excessive axial forces applied to the catheter, in the distal direction, are distributed over a surface area of the distal end region proximal the distal tip and are prevented from concentrating at the distal tip of the catheter. The invention fulfills these needs and others.